1. Field of the Invention
The present invention relates to an apparatus and method for making a slotted rod which is accommodated in an optical cable and protects a plurality of optical fibers.
2. Related Background Art
Employed as a slotted rod for an optical cable is a slotted rod S (spacer) shown in FIG. 1 in which a slot G accommodating an optical fiber is formed to meander as a so-called SZ groove in order to prevent extraordinary tension from being applied to the accommodated optical fiber and improve workability for taking out the optical fiber for branching. In general, such a slotted rod S is manufactured in a manner that, while a molten resin is extruded around a tension member L0 by an extruder through its rotatable die, the die is rotated alternately in reverse directions to form the above-mentioned slot G in the outer peripheral face of the resin layer thus extruded.
In the above-mentioned method, however, due to the alternate reversing operation of the rotatable die (operation for alternating its rotational direction), the tension member L0 maybe twisted, thereby the reversing angle .theta. of the slot G in thus molded slotted rod S (the center angle formed on the slotted rod cross-section corresponding to the slot formed in a cycle of reversing rotation of the rotatable die(i.e. a period between any two consecutive reversions of the rotational direction of the die); see FIG. 2) may become smaller than the rotational angle of the rotatable die (the angle formed by a cycle of reversing rotation of the rotatable die). Consequently, in order for the slot G to attain a desired reversing angle, the rotatable die must have a large rotational angle, thus making it difficult to improve the manufacturing speed that depends on the speed of the alternate reversing operation. Here, in FIG. 2, P is a pitch for a cycle of reversing rotation of the rotatable die and corresponds to the distance which the tension member L0 passes during such a single cycle (hereinafter referred to as a `reversing pitch`).
Known as the manufacturing apparatus having a rotatable die which alternately reverses its rotational direction are those disclosed in Japanese Patent Application Laid-Open No. 3-110509 and No. 1-303408. Japanese Patent Application Laid-Open No. 3-110509 discloses a manufacturing apparatus in which, for restraining the tension member from being twisted, the tension member is grasped upstream the rotatable die. Japanese Patent Application Laid-Open No. 1-303408 discloses a manufacturing apparatus in which, in the state where the tension member is grasped upstream the rotatable die, the grasping unit is synchronized with the reversing operation of the rotatable die such as to twist the tension member in the same direction as the rotation of the rotatable die.